An optical access network, called “long-reach passive optical network (PON)”, is described as a next-generation optical transmission system in G. Talli et al., J. Lightw. Technol., Vol. 24, No. 7, 2827-2834 (2006). The long-reach PON is a transmission system for transmission of signal light from an optical line terminal (OLT) at a telephone central office through a splitter to an optical network unit (ONU) at a subscriber's home, and enables an optical transmission line between the OLT and the ONU to have a long length and can therefore reduce communication cost.
Such an optical transmission system preferably uses a low-attenuation optical fiber in order to extend a communication distance while maintaining an optical signal-to-noise ratio (OSNR). As regards a low-attenuation optical fiber, an optical fiber including a pure-silica-core is disclosed in T. Kato et al., Electron. Lett., Vol. 35, No. 19, 1615-1617 (1999). Unfortunately, the introduction of pure-silica-core optical fibers to optical access networks has not progressed for economic reasons that a typical pure-silica-core optical fiber is expensive.
S. Sakaguchi et al., Appl. Opt., Vol. 37, No. 33, 7708-7711 (1998) and JP2006-58494A disclose technology for reducing attenuation in a general-purpose optical fiber compliant with ITU-T G. 652. According to this technology, when an optical fiber preform is drawn to form an optical fiber, the optical fiber is slowly cooled to reduce the fictive temperature of glass constituting the fiber in order to reduce Rayleigh scattering in the optical fiber, thus achieving low attenuation.